1. Field of the Invention
This invention relates to methods and apparatus for reading identification marks on semiconductor wafers.
2. Introduction to the Invention
It is important to be able to identify semiconductor wafers (i.e. wafers composed of silicon or other semiconductor material such as gallium arsenide) throughout the different treatments which they undergo before being diced into chips. For this purpose, it is conventional for each wafer to carry an identification mark (usually a unique alpha-numeric mark) at a standard location on one of its principal surfaces, usually adjacent a flat, notch or other identifiable discontinuity formed on the periphery of the wafer so that the orientation of the wafer can be easily determined. Many of the treatments undergone by the wafer after it has been marked involve the deposition of metals or chemicals over the mark. The mark must, therefore, be made by a method which deforms the smooth surface of the wafer, e.g. by means of a laser or a diamond stylus, so that the mark will show through such deposited layers. The term "incised" is used in this specification to denote a mark made by any such method. The marks are very small in size, such that magnification is a practical requirement for reading them, and even though the marks are incised on the wafer, they become more and more difficult to read as the wafer is processed. As a result, the semiconductor industry has not been able to develop a reliable method for machine reading identification marks on wafers. Such a method is highly desirable in order to achieve the objectives of reducing the misidentification of wafers, increasing production rates, increasing automation, and minimizing handling of the wafers.